The present invention relates to a new and improved method of producing structural insulation containing approximately fifty or more percent xonotlite.
Calcium silicate type thermal insulation materials have been made for a great number of years and are of two general crystalline types, i.e. xonotlite and tobermorite. Tobermorite has five molecules of water per molecule of calcium silicate, while xonotlite has only one molecule of water per molecule of calcium silicate. Xonotlite can stand temperatures of approximately 1,800.degree. F. while tobermorite is limited to a range of approximately 1,200.degree. F. Xonotlite materials require more autoclaving to produce and are more costly so that by far the bulk of the inorganic insulation materials which have been produced were of the tobermorite type. Xonotlite and tobermorite insulation materials are made from water rich slurries of lime, silica, fillers, and fibrous reinforcements which are caused to thicken into a gel that holds the excess water suspended. The thickened materials are either poured into molds to precrystallize or are precrystallized and then filtered into the desired finished shape. The precrystallized or prehardened materials are then placed into an autoclave where they are heated under pressure to temperatures above 350.degree. F. to produce the final crystalline structure, following which the excess water is evaporated to leave a lightweight porous insulation material. It is imperative that the slurry thickens into a gel to hold excess water uniformly throughout the materials, which when evaporated, gives voids therein to increase the insulating value and lighten the product. The gel formation keeps material from shifting in the molds in the pan cast process and helps cause filter pressed material to retain their shape so that they can be handled and put into an autoclave.
Up until recently, asbestos has been the only material that has been used as a reinforcement, because the highly branched nature of asbestos suspends the solids in the slurry better than any known material. Recently, there has been efforts to produce the insulation without the asbestos. The Helser-Shannon U.S. Pat. No. 3,902,913 teaches how an insulation material can be made by using a combination of wood pulp, alkali resistant glass fibers, and diatomaceous earth.
According to principles of the present invention, it has been determined that xonotlite does not form in the presence of free aluminum ions. It is believed that calcium aluminum silicates will not form hydrates having but a single crystal of water, and that this may account for the reason that aluminum salts, or materials containing aluminum ions, poison the reaction to xonotlite. Diatomaceous earth and clay produce a thickening or a gelling reaction. It has been determined that diatomaceous earth, and clay, contain sufficient free aluminum ions that they poison the xonotlite reaction. Prior to the present invention, therefore, there was no known way of thickening or pregelling a xonotlite forming slurry at ambient conditions. According to principles of the present invention, a process has now been provided for making xonotlite without using asbestos, or gel forming ingredients that poison or inhibit the formation of xonotlite crystals.